This invention relates to an optical filter. It may be used, for example, in spectroscopic applications, such as Raman spectroscopy.
In Raman spectroscopy, a sample is illuminated with monochromatic light, e.g. from a laser. The resulting scattered light is analysed spectroscopically. Most of the scattered light is Rayleigh scattered, at the same wavelength as the exciting laser beam. However, a minor proportion is scattered at Raman-shifted wavelengths as a result of interactions with molecules making up the sample. Different molecular species have different characteristic spectra of such Raman-shifted light, and so this Raman effect can be used to analyse the molecular species present.
In order to analyse the Raman spectrum, it is first necessary to remove the much stronger Rayleigh scattered light, which would otherwise completely swamp the Raman scattered light.
A known Raman spectroscopic apparatus is described in European Patent Application No. EP-A-543578 (Renishaw). This uses a holographic notch or edge filter to reject the Rayleigh scattering, while transmitting the Raman scattering. With the arrangement described, it is possible to detect Raman scattered light with wavenumbers as low as 50 cm from the Rayleigh line. However, detecting Raman scattered light closer to the Rayleigh line than about 50 cm would require a filter with a steeper transition in its transmission characteristic, between a high optical density at the laser wavelength and a low optical density at the Raman wavelengths which are to be transmitted.
Traditionally, double and triple monochromators have been used in order to separate very low Raman wavenumbers from the Rayleigh scattered light at the laser wavelength. A conventional double monochromator comprises two dispersive stages, each comprising a dispersive element (usually a diffraction grating). An entrance slit is provided before the first dispersive stage, and an intermediate or transfer slit between the two stages. A spectrum dispersed by the first stage is focused in the plane of the intermediate or transfer slit, which transmits just one monochromatic wavenumber from the spectrum into the second dispersive stage. The wavenumber thus transmitted can be tuned by moving the slit in the plane of the focused spectrum, or more normally by rotating the grating or other dispersive element. The second dispersive stage may be xe2x80x9csubtractivexe2x80x9d, i.e. it may reverse the dispersion in the first stage.
A disadvantage of a monochromator filter of this type is that only one Raman wavenumber is transmitted at a time. Analysing a full Raman spectrum requires that the filter should scan the spectrum, which is time consuming.
It would of course be possible to use a wider intermediate or transfer slit, passing a range of Raman wavenumbers. See for example U.S. Pat. No. 5,424,825 (assigned to Dilor). Here, there is a relatively wide aperture in the plane of the focused dispersed spectrum, passing a wider band of wavenumbers to the second dispersive stage (which is said to be subtractive and which therefore re-forms a polychromatic beam). A narrow light trap is provided within the aperture, to block a narrow band around the Rayleigh scattered line. U.S. Pat. No. 5,424,825 also suggests an inverse arrangement, having a plane mirror in place of this relatively wide transfer aperture. The width of this transfer mirror corresponds to the width of the aperture, and all Raman wavenumbers falling on the mirror within this range of wavenumbers are reflected towards the second dispersive stage. A narrow slit in the transfer mirror allows a narrow waveband around the Rayleigh scattered line to pass through the mirror, without being reflected into the second dispersive stage; this corresponds to the narrow light trap in the transfer aperture.
A somewhat similar filter having a transfer mirror of a defined width (but not for Raman spectroscopy) is described by W D Wright, Optica Acta, 1, 102-107 (September 1954). In this paper, the first dispersive stage takes the form of the upper parts of two glass prisms, in series. The resulting dispersed spectrum from these is focused onto a strip of spherical mirror. The radius of curvature of the mirror is chosen so as to return the spectrum back through the lower parts of the prisms, which thus form the second, subtractive dispersive stage. By choosing an appropriate width for the strip of spherical mirror, the spectral bandwidth to be returned through the second dispersive stage can be arbitrarily selected.
U.S. Pat. No. 3,865,490 (Grossman) shows another such arrangement with a transfer mirror, intended for Raman spectroscopy and using a diffraction grating instead of prisms. The transfer mirror has a narrow slit which removes the Rayleigh line. Once again, the mirror has a radius of curvature chosen to ensure that the reflected rays are correctly directed towards the second, subtractive dispersive stage.
Reference has been made above to the plane into which the dispersed spectrum is focused. However, in general the locus of the focused spectrum does not lie in a single plane. Rather, this locus is curved. Moreover, the curve does not in general coincide with the curvature required of the mirrors in the Wright paper and the Grossman patent, in order to direct the rays towards the second dispersive stage.
This curvature of the locus of the focused spectrum does not give rise to a problem in a conventional monochromator having only a narrow transfer slit. In such a case, it is easy to arrange that the slit coincides with the focused spectrum. However, where an aperture or mirror of a substantial width is used, the focus does not coincide with the aperture or mirror over its full width. This results in significant aberrations, especially where a wide bandwidth is desired.
Of the above prior art documents, only the Grossman patent recognises this problem and proposes a solution. However, this solution involves a mirror made up of numerous segments. The faces of each segment are ground with the radius of curvature required in order to return the rays correctly into the second dispersive stage. They are then displaced into a stepped or echelle arrangement, corresponding to the locus of the spectral image. Such an arrangement is complicated to manufacture and set up, and a different stepped or echelle mirror configuration is required if the filter is to be re-tuned to reject a different laser wavelength.
A further problem with the above prior proposals relates to the steepness of the transition in optical density, between a high optical density at the wavelength(s) which are to be rejected, and a low optical density at the wavelengths which are to be transmitted. In the case of Raman spectroscopy, this steepness governs how close to the laser line the Raman spectrum can be discriminated, i.e. whether very low Raman wavenumbers can be detected. Imperfections in the edge of the transfer aperture or mirror mean that a steep transition can only be obtained by using very high dispersion. However, in these prior proposals, the transfer aperture or mirror can only be a finite size (in order to minimise aberrations). The result of such high dispersion over a transfer aperture or mirror of finite size is that the filter can have only a relatively narrow bandwidth (only a narrow band of wavelengths can be passed to the second dispersive stage).
In one aspect, the present invention provides an optical filter comprising:
a first dispersive stage,
a second, dispersive stage,
a transfer mirror or aperture between the first and second dispersive stages,
each dispersive stage comprising a dispersing element and focusing means, having a predetermined focal locus for dispersed light,
light from the dispersing element in the first dispersive stage being focused by the focusing means of the first dispersive stage to produce a spectral image in the focal locus of the first dispersive stage,
the transfer mirror or aperture being located in said focal locus, light of said spectral image which falls upon the transfer mirror or aperture being accepted and passed to the second dispersive stage, and light of said spectral image which does not fall upon the transfer mirror or aperture being rejected,
light from a spectral image in the predetermined focal locus of the second dispersive stage being recombined subtractively by the focusing means and dispersing element thereof into a polychromatic beam,
wherein the predetermined focal loci of the first and second stages are coincident, the transfer mirror or aperture being located in said coincident loci.
Preferably the dispersing elements of the first and second stages are imaged upon one another. A field optic (e.g. a lens or a non-planar mirror) between the two dispersive stages may co-operate with the focusing means of the two stages to achieve this. The dispersing elements are preferably diffraction gratings.
We presently prefer that the focusing means in each dispersive stage should be concave mirrors. However, a focusing lens may be used instead; or a concave diffraction grating may provide both the dispersing element and the focusing means.
In a second aspect, the present invention provides an optical filter comprising:
a first dispersive stage,
a second dispersive stage,
a transfer mirror or aperture between the first and second dispersive stages,
each dispersive stage comprising a dispersing element (such as a diffraction grating) and a concave mirror, light from the dispersing element in the first dispersive stage being focused by the concave mirror of the first dispersive stage, producing a spectral image in a predetermined locus, the transfer mirror or aperture being located in said locus, light of said spectral image which falls upon the transfer mirror or aperture being accepted and light of said spectral image which does not fall upon the transfer mirror or aperture being rejected,
the concave mirror of the second dispersive stage receiving the light which is accepted by the transfer mirror or aperture and passing it to the dispersing element of the second dispersive stage,
characterised in that the concave focusing mirror in at least one of said dispersive stages has a radius of curvature which is centred on the dispersing element of that dispersive stage.
It is then possible for the transfer mirror or aperture to be located in a manner which both correctly transfers the light from the first dispersive stage to the second dispersive stage, and also coincides with the locus of the focus of the spectral image. In the case of a transfer mirror, it may suitably be convex in order to achieve this. Preferably, the concave focusing mirror of both said dispersive stages has a radius of curvature which is centred on the dispersing element of the respective dispersive stage.